The invention relates to a process for the preparation of a heat curable paint binder composition containing a compound comprising hydroxyalkylamide groups. The invention further relates to a paint composition comprising such a binder composition, a coating and a coated substrate.
The use of a compound comprising hydroxyalkylamide groups in a powder paint binder composition is disclosed in, for example, WO 99/16810. This publication discloses a polymer containing ester groups and at least one amidegroup in the backbone, having hydroxyalkylamide end groups and having a weight average molecular mass of xe2x89xa7800 g/mol. Said polymer can, for example, be obtained by reaction in a polycondensation reactor of a cyclic anhydride and an alkanolamine after which the compound comprising hydroxylamide groups, being a polyesteramide, is obtained through polycondensation.
Also EP-A-0.322.834 discloses a powder paint binder composition comprising a compound containing a hydroxylamide group as a crosslinker. This compound is obtained by reaction of the dimethyl ester of adipic acid and diethanolamine after which the reaction mixture is heated and methanol has to be removed by destination or fractionation.
It is a disadvantage of the process according to WO 99/16810 (and also of EP-A-0.322.834) that the process for the preparation of the compound comprising hydroxyl amide groups consists of two unit operations. First the raw materials have to be contacted with each other, heated when desired. Secondly the mixture is heated to form a polymer through polycondensation whereby volatile condensation products, for example water or methanol, have to be removed by distillation. Such a compound was used in a paint binder composition, by mixing and extruding of the compound with a polymer.
For a long time intensive efforts have been made to develop crosslinkers and polymers for thermosetting powder paint compositions which have to be cured to obtain powder coatings. Attempts are still being made to find binder compositions for thermosetting powder paints with a good flow behaviour, good storage stability and a good reactivity. A thermosetting powder paint binder composition generally contains more than 50 wt.% polymer and less than 50 wt.% crosslinker. A coating that is ultimately obtained with a powder paint must meet many varying requirements. Various systems are known. Some systems release volatile components during the curing. These systems present the drawback that they form coatings with bubbles and/or that undesirable emissions are released. As far as the latter is concerned, the volatile component, if organic in origin, may cause undesirable environmental or health problems. It has moreover been found that all the desired properties of the powder paint or powder coating are not always realized.
In other systems use is made of polyesters and crosslinkers containing an epoxy group. No volatile components are generally released from these systems. However, the use of bisphenol-A-epoxy resins in the so-called hybrid systems results in coatings that exhibit a relatively great extent of yellowing and powdering when exposed to UV light, while the frequently used triglycidyl-isocyanurate (TGIC) crosslinker is toxicologically suspect.
With respect to the crosslinkers there is a continuous effort to provide economic processes for the preparation of useful crosslinkers which result in required properties of the powder paint and powder coating such as for example non-reactivity, powder stability, good mechanical properties, high weatherability and a high blister limit. Blistering is the formation of dome-shaped, liquid or gas-filled projections in a film resulting from local loss of adhesion and lifting of the film from the underlying surface or coating. The blister limit is the maximum thickness of the film which does not contain blisters.
It is the object of the present invention to provide an improved economical process for the preparation of a heat curable paint composition containing a compound comprising hydroxyl amide units.
The process according to the invention is characterised in that at first a compound comprising hydroxyalkyl amide units and carboxylic acid units is obtained by reacting a cyclic anhydride and an alkanolamine in a mixing device and that secondly a binder composition is obtained by mixing the compound and a polymer in a second mixing device.
During the curing process of a powder paint system comprising a hydroxyalkylamide crosslinker and a polymer containing carboxylic acid groups, water is released during the film formation. This phenonenon may lead to a low blister limit ( less than 100 xcexcm). If the crosslinker contains also carboxylic acid groups, as in the present invention, the extra release of reaction water as a result of the reaction between the reactive groups of the crosslinker itself has to be taken into account. Therefore, a low blister limit could be expected in compositions in which the total amount of carboxylic acid groups in the binder system is selected close to the total amount of hydroxyalkylamide groups in the binder system.
The product obtained with the process according to the invention contains both hydroxyalkylamide groups and carboxylic acid groups. Suprisingly, a powder paint composition comprising this compound results in a high blister limit ( greater than 120 xcexcm).
The mixture after the first mixing step generally consists of bis(hydroxylamide) functional carboxylic acids, alkanolamine salts thereof and oligomers comprising ester and amide bonds. This mixture generally has a weight average molecular weight between 100 and 1000, and preferably between 200 and 800. The resulting compound may comprise less than 30% by weight of oligomers having ester and amide bonds and having a weight average molecular weight between 100 and 1000.